This invention relates to the field of polymer films containing migratory additives that can be laminated to other films and methods for forming such films. In particular, this invention relates to a film having a first outer layer containing a migratory additive, such as silicone oil, and a second outer layer containing a metallocene polyethylene that can be laminated to another film structure.
Multiple layer films are commonly used in the packaging industry for a variety of food products, such as potato chips, cookies, candies and other snack foods. These films can be formed by coextrusion methods or by laminating two or more films together. Many laminated films use oriented polypropylene (OPP) in the core layer because of its durability and strength. Different surface layers are selected for the interior and exterior layers of the package that are suitable for the particular product. For example, chips may have one type of inner layer, while wrapped candies may have a different type of inner layer. Similarly, the outer layer may be selected to enhance printability, clarity, gloss or other characteristics of the film. Frequently, additives are used to impart the desired characteristics to the outer layer.
The use of migratory additives to impart specific properties to the outer layers of multiple layer packaging films to improve specific film characteristics is well known in the art. These additives are often added to the thermoplastic resins before the film is extruded and only migrate to the surface (i.e., bloom) after the film is formed. When these migratory additives bloom to the surface of the outer layer, they promote the desired characteristics on the film surface. Often, the additive is a silicone oil. Silicone oils are added to an exterior layer of a film to improve its coefficient of friction in the heated state and reduce its slide resistance which occurs during hot plate seal by an automatic packaging machine. Examples of silicone oils which are useful for this purpose are olefin-modified silicone, polydimethylsiloxane, polymethylphenylsiloxane, polyether (e.g., polyethylene glycol, polypropylene glycol)-modified silicone, olefin/polyether-modified silicone, epoxy-modified silicone, etc.
The presence of a migratory additive on the surface of an exterior layer of a film can create problems when the film is used for an application that requires the opposing, untreated exterior surface of the film to be devoid of the additive. The untreated exterior surface is contacted by the additive when the film is rolled and subsequently unrolled. Rolling the film brings the treated exterior surface in contact with the untreated exterior surface and the additive is transferred. In order for the film to be used in an application that requires the untreated exterior layer to be devoid of the additive, the additive must first be removed. When the migratory additive is a silicone oil that has been added to improve the film's coefficient of friction and reduce its slide resistance, the characteristics that are desirable for the treated exterior surface are undesirable for the untreated exterior surface. Silicone oil on the untreated exterior surface is especially undesirable when the surface is to be laminated to another structure.
The key step in laminating is the creation of strong adhesive bonds between the films. To assure the formation of strong adhesive bonds, the adhesives must be compatible with the materials on the surfaces of the opposing films. While silicone oils impart useful properties to a film, film surfaces containing silicone oil have been found to have poor adherence when laminated to another film structure. In the past, this problem has been overcome by applying a primer (typically in the form of a solvent) to the film prior to lamination. In addition to the cost of the solvent, this method adds an additional step to the manufacturing process and is, therefore, more costly and undesirable. This has created a need for a film that does not require the application of a primer to neutralize silicone oil prior to lamination.
The films of the present invention find their principal utility as a structure that can be laminated to other film layers. However, it is to be understood that these films can also be used independently as a stand alone packaging material without adding additional layers.